This invention relates generally to an improved level indicator for stored free-flowing material. More particularly, this invention relates to a level sensing device for sensing and displaying the level of free-flowing material (whether conductive or non-conductive in nature) stored within a storage compartment at a point relatively remote from the compartment itself.
Level gauges or sensing devices which indicate at remote points the detected level of material within storage compartments have long been known, but such gauges or detectors have been typically limited in the type of material whose level they can practically detect. Many level gauges require that the material whose level is detected be in electrical contact with the gauge itself, and some require that the material itself be electrically conductive.
Some examples of prior art level indicators are
______________________________________ U.S. PAT. NO. INVENTOR(S) DATE OF ISSUE ______________________________________ 4,165,641 Pomerantz et al. August, 1979 3,935,739 Ells February, 1976 3,862,571 Vogel January, 1975 3,321,971 Llewellyn et al. May, 1967 3,025,464 Bond March, 1962 3,010,320 Sollecito November, 1961 2,868,015 Haropulos January, 1959 2,751,531 Barrett June, 1956 ______________________________________
In Haropulos, U.S. Pat. No. 2,868,015, a capacitive step level indicator for conductive liquids uses a plurality of vertically stacked capacitive elements in a tank, each energized through a respective relay. A plate in the storage tank completes a circuit through the received liquid. When fuel fills the tank, current flow through any relay is too small to operate it, and a bank of indicator lights remain illuminated ("tank full"). As fuel is withdrawn, conductive sea water replaces it and conducts adequate current to activate a relay and extinguish its corresponding lamp for indicating a diminished level of stored fuel.
An inherent drawback generally in such system is that it is operable only with electrically conductive filler fluids (i.e., the fluid which replaces the fuel), and is potentially dangerous with combustible fluids such as gasoline or the like.
Another approach is set forth in Ells, U.S. Pat. No. 3,935,739, wherein a liquid level gauging apparatus includes an upright conducting probe adapted to be immersed in a liquid adjacent to a series of vertically spaced electrodes, closely adjacent to the probe itself. Each of the electrodes constitutes one plate of a capacitor, with the other plate formed by the single upright conducting probe. When a particular electrode is above the level of fluid in a tank, the magnitude of the A.C. signal received at that electrode is of a significantly smaller magnitude, which provides only a low level drive voltage to its respective comparator amplifier. The device has a circuit which sums the signals received from each of the capacitor plates to generate a meter reading which corresponds to the number of electrodes submerged in the stored liquid at a particular time, and thereby provides an indication of the liquid level in the tank.
The Ells device apparently requires that the stored fluid be in contact with his electrodes, and that the material be a liquid which has a dielectric constant greater than air. Contact could prove hazardous if used with combustible materials.
Still another approach is found in Vogel, U.S. Pat. No. 3,862,571, which discloses a multi-electrode capacitive liquid level sensing system which also apparently requires electrical contact between a series of probes and the fluid whose level is being detected. Preferably, at least three probes of different lengths extend to different depths within the liquid itself.
U.S. Pat. Nos. to Sollecito, 3,010,320; Bond, 3,025,464; Llewellyn et al., 3,321,971; and Barrett 2,751,531 each generally relate to level measuring devices for liquids and each utilize electrical contact between the liquids and portions of their various sensing devices themselves; hence, such devices generally suffer from the same drawbacks noted above with reference to the other patents requiring electrical contact between the fluids being measured and the various probe members.
Another liquid level sensing means is found in Pomerantz et al., U.S. Pat. No. 4,165,641. While direct electrical contact is not apparently required between the probe and the liquid or fluid whose level is being determined, the fluid whose level is being detected must be conductive to some extent, such as a fluid having an electrical resistivity of up to 20 megohm-centimeters. Thus, this device is only operable with limited types of material such as at least partially conductive materials.